Check if a Local Port is Available for Use

#!/bin/bash

# Usage: ./check_port.sh <port_number>

PORT="$1"

if [ -z "$PORT" ]; then
  echo "Usage: $0 <port_number>"
  exit 1
fi

# Validate if PORT is a number
if ! [[ "$PORT" =~ ^[0-9]+$ ]]; then
  echo "Error: Port number must be an integer." >&2
  exit 1
fi

# Check if the port is in use using lsof (preferred on Linux/macOS) or netstat

if command -v lsof &> /dev/null; then
  # Using lsof: -i :<port> lists network files; -sTCP:LISTEN filters for TCP sockets in LISTEN state.
  if lsof -i :"$PORT" -sTCP:LISTEN &> /dev/null; then
    echo "Port $PORT is IN USE."
    exit 1
  else
    echo "Port $PORT is AVAILABLE."
    exit 0
  fi
elif command -v netstat &> /dev/null; then
  # Using netstat: -t (TCP), -u (UDP), -l (listening), -n (numeric). grep -q suppresses output.
  if netstat -tuln | grep -q ":$PORT\b"; then # \b ensures whole word match for the port
    echo "Port $PORT is IN USE."
    exit 1
  else
    echo "Port $PORT is AVAILABLE."
    exit 0
  fi
else
  echo "Error: Neither 'lsof' nor 'netstat' found. Cannot check port status." >&2
  exit 1
fi